(1) Field of the Invention
The present invention relates to a pitch stabilizer with box configuration suitable for providing stability to aircrafts, and in particular to rotorcrafts.
Conventionally, a rotorcraft comprises an airframe extending longitudinally from the nose of a fuselage to a vertical fin, transversally on either side of a vertical mid-plane, and in a vertical direction from a bottom portion fitted with landing gear to a top portion fitted with a main lift and propulsion rotor. The fuselage can comprise a substantially longitudinal tailboom carrying a fin at its end.
(2) Description of Related Art
A rotorcraft generally comprises stabilizer surfaces. The rotorcraft may be provided with a yaw stabilizer surface in order to compensate, at high speed, the torque exerted by the main rotor on the fuselage.
Similarly, a helicopter sometimes has a pitch stabilizer comprising, for example, two pitch stabilizing surfaces extending symmetrically on either side of the vertical mid-plane. Such pitch stabilizer is often referred to as a “horizontal stabilizer”.
The flow of air induced by the main rotor of the rotorcraft is known as downwash. The impact of the downwash on the pitch stabilizer causes several difficulties which are especially relevant in the transition between hover flight and high speed cruise conditions. During this transition phase, the pitch stabilizer experiences great changes in the induced flow field, which implies variations in the forces exerted on the stabilizer and in the aerodynamic efficiency.
Particularly, the variation of forces can lead, when the main rotor head is articulated, to a change in the fuselage pitch attitude known as hump phenomenon. When the main rotor head is bearingless, said variation of forces can lead to a notable increase in the rotor mast moments known as pitch-up problem.
Some prior art documents have already considered such problems, and among the proposed solutions there are some disclosing bi-plane pitch stabilizers.
In document EP0229896, two separate pitch stabilizers are located at the aft of the tailboom. One stabilizer is located on top of the fin, the other on its lowest position. The patent does not deal with the pitch-up problem or the hump phenomenon.
Documents U.S. Pat. No. 2,369,652 and GB606420 disclose a bi-plane pitch stabilizer comprising an upper and a lower horizontal surface.
Document US20120018570 also describes a method of minimizing the hump phenomenon by means of a bi-plane pitch stabilizer.
Document U.S. Pat. No. 3,902,688 describes a rotorcraft pitch stabilizer configuration that has for object to improve the pitch stability. The two pitch stabilizers are mounted at the upper and lower tips of a vertical fin, which extends above and below the tailboom so that the stabilizers are located outside the fuselage wake in forward flight. The two pitch stabilizers do not affect each other aerodynamically.
Differently to those bi-plane configurations, document U.S. Pat. No. 3,706,432 discloses a pitch stabilizer unit mounted on the tailboom and which includes a torque tube mechanism.